Automatic slide-changing mechanism

ABSTRACT

A slide-changing mechanism for use in a photographic slide projector and including an electric motor for changing slides under both manual and timed automatic control. Manual control is effected by a switch which energizes the motor for rotation in a selected direction for one cycle that is terminated by a switch opened by a cam turned by the motor, and automatic control is effected by a silicon-controlled rectifier triggered by a relaxation oscillator circuit including a potentiometer for varying the rate of charge of a timing capacitor in the circuit. The cam-operated switch controls the power supply of the timing circuit to disconnect the circuit during changing of slides, and resets the circuit substantially to initial conditions preparatory to timing the next interval.

United States Patent 3,498,703 3/ i970 Gerry Primary Examiner-LeonardForman Assistant ExaminerSteven L. Stephan Attorney-Fulwider, Patton,Rieber, Lee & Utecht ABSTRACT: A slide-changing mechanism for use in aphotographic slide projector and including an electric motor forchanging slides under both manual and timed automatic control. Manualcontrol is effected by a switch which energizes the motor for rotationin a selected direction for one cycle that is terminated by a switchopened by a cam turned by the motor, and automatic control is effectedby a silicon-controlled rectifier triggered by a relaxation oscillatorcircuit including a potentiometer for varying the rate of charge of atiming capacitor in the circuit. The cam-operated switch controls thepower supply of the timing circuit to disconnect the circuit duringchanging of slides, and resets the circuit substantially to initialconditions preparatory to timing the next interval.

AUTOMATIC SLIDE-CHANGING MECHANISM BACKGROUND OF THE INVENTION Thisinvention relates generally to slide-changing mechanisms for slideprojectors, and more particularly to an automatic slide-changingmechanism including an automatic timer for controlling successive timedintervals between slide changes.

In the course of the development of photographic slide projectors,mainly those for use in the home, the changing of the slide in theprojector has been partially automated by utilizing mechanisms driven byelectric motors. To provide for the automatic changing of a slide aftera preset viewing time, in addition to the usual manual actuation ofslide changes, automatic timers have been incorporated in such slideprojectors to activate the slide-changing mechanism periodically afterdwell periods of selected length. Heretofore, the best of the automatictimers have principally employed thermoelectrically actuated time-delayswitches.

However, the adjustment and operation of a thermoelectrically operatedtiming device has been somewhat unreliable in that it is largelydependent upon mechanical construction tolerances and is sensitive toheat variations in and around the projector. Additionally, suchthennoelectric timing devices require numerous relatively complex,interconnected mechanical elements which must be built into the slideprojector, so that such devices have been relatively expensive toconstruct and install.

SUMMARY OF THE INVENTION The present invention resides in an automatictimer for a slide projector in which the components of a manuallyactuated slide-changing mechanism are combined in a novel manner withsimple and inexpensive electronic components to produce an automatictimer that is highly effective, easily adjustable as to the timed dwellinterval between slide changes, and very compact in construction,primarily as a result of the dual utilization of components of themanually actuated mechanism. More specifically, the preferred embodimentof the automatic timer includes a motor-driven cam and camoperatedswitch for terminating operation of a slide-changing motor at the end ofeach cycle, a silicon-controlled rectifier in parallel with the motorswitch for effecting manual starting of the motor, and a relaxationoscillator circuit for triggering the controlled rectifier periodically,after a selected and adjustable dwell, to energize the slide-changingmotor for a cycle that is terminated by the cam-operated switch. Thecam-operated switch controls the power applied to the relaxationoscillator circuit as well as the length of the slide-changing cycle,and makes it possible to utilize an extremely simple timing circuit toperform the timing and adjustment functions that heretofore requiredmuch more complex and expensive timing components.

BRIEF DESCRIPTION OF THE DRAWING The drawing shows a partially exploded,pictorial, mechanical view of the basic parts of the slide-changingmechanism of a slide projector together with an electrical schematicdiagram of the control circuit, the mechanism and circuit being combinedin accordance with the present invention.

DETAILED DESCRIPTION The preferred embodiment of the timing circuit ofthe present invention is particularly adaptedfor use with aslidechanging mechanism for a slide projector such as that shown in thedrawing. The illustrated mechanism 10 is basically of a type describedin US. Pat. No. 3,l70,369 entitled SLIDE PROJECTOR, issued on Feb. 23,1965, to G. .I. Frey et al., to which reference is made for details ofconstruction and operation not contained herein.

To aid in understanding the present invention, however,'the basicoperation of the slide-changing mechanism 10 will be briefly described.Generally, the slide projector utilizesa slide magazine (not shown) inwhich individual slides 12 are mounted in clips 14 on an endless belt 16which travels around spaced upper and lower shafts I8, 20 set in themagazine. The magazine is removably mounted on the slide projector sothat a depending slide 12 is brought into position for projectionbetween a lamp 22 and a lens assembly 24 each time the belt is advancedone step through a preselected increment of feed.

In the illustrated slide-changing mechanism 10, the lower shaft 20 isdriven by a drive wheel 26 through a clutch assembly 28. The drive wheel26 has four notches 30 equally spaced about its periphery to be engagedby a pin 3] on the free end portion 32 of a drive arm 33. The oppositeend 34 of the drive arm is pivotally connected to the periphery of acrank disc 36 driven by an electric motor 38 that preferably isreversible to provide for reversible feeding of the slides 12.

The electrical control for the mechanism 10 is such that theslide-changing cycle can be started by momentarily depressing a manualslide-change switch SW which starts the motor 38. As the crank disc 36is rotated by the motor 38, in the direction indicated by the arrow 39,drive arm 33 slides along a guide 40 so that the free end 32 of the armfirst moves to the right, over and above the periphery of the drivewheel 26, and then pivots about the guide to drop the pin intoengagement with the notch 30 positioned near the top of the wheel. Asuitable spring (not shown) holds the arm in sliding engagement with theguide 40 at all times.

As the crank disc 36 continues to rotate, the pin 31 is shifted back tothe left to turn the drive wheel 26 through a preselected arc, herein toadvance the belt a corresponding amount and move a new slide 12 into theprojection position at the bottom of the magazine. As the crank disc 36completes one revolution, the pin 31 is lifted out of the notch 30 todisengage the arm 33 from the wheel. A cam 46 on the periphery of thedisc 36 cooperates with a cam-operated switch SW to deenergize and stopthe motor 38 as each slide change is completed. f

To position each slide 12 positively for projection, a fork 48 isprovided to engage a lower edge 50 of the slide from below, and pressthe slide firmly against the lower shaft 14. The positioning fork is onthe end of an arm 52 carried on a pivot 54 at its other end, and swingsupward into locking engagement with a slide 12 when the arm is raised bya solenoid 56. During the slide-changing cycle, the solenoid 56 isdeenergized and the arm 52 thus is lowered to hold the fork out ofengagement with the slides. When a new slide is in the projectionposition, the solenoid is energized to raise the fork into positioningengagement with the slide. The motor 38 and solenoid 56 of theillustrated slide-changing mechanism 10 are operated from a suitable DCpower supply preferably derived from a l 15 volt AC utility line bymeans of the usual stepdown transformer and rectifier-filter combination(not shown).

In the control circuit for manual operation, a positive side 58 of thepower supply is connected to both a positive terminal 60 of the motor 38and one terminal 62 of the solenoid 56. A negative side 64 of the powersupply is connectable alternately to a negative terminal 66 of the motor38 and to another terminal 68 of the solenoid 56 by means of the camswitch SW which herein is a single-pole, double-throw switch. The motorand solenoid terminals 66 and 68 are connected to respective first andsecond outer contacts 70 and 72 of switch SW for alternate engagementwith the center contact 74 of the switch.

The center contact 74 is connected to the negative side 64 of the powersupply, and the position of the center contact is determined by a camfollower 76 which is positioned beside the periphery of the crank disc36, herein at the bottom thereof. In the dwell position of the disc 36,the cam 46, which is a lobe on the periphery of the disc, engages thefollower 76 and holds the center contact 74 of the cam switch SW incontact with the switch contact 72 connected to the solenoid terminal68, energizing the solenoid 56 to move the positioning fork 48 upwardlyinto the raised position. During the slidechanging cycle, the cam lobe46 moves away from the follower 76 and the center contact 74 moves awayfrom switch contact 72 to deenergize the solenoid and lower the fork.

A single-pole, single-throw switch, preferably of the pushbutton,momentary-contact type, serves as the manual slidechange switch SW, andis connected between the negative side 64 of the powerline and thenegative terminal 66 of the motor 38. When it is desired to change aslide 12, the manual switch SW, is momentarily depressed, connecting themotor 38 across the power supply terminals 58 and 64. As the motor 38begins to turn, the cam lobe 46 moves out of engagement with thefollower 76 to deenergize the solenoid 56, the center contact movinginto engagement with the alternate contact 70 of switch SW The motor 38then is connected to the negative side 64 of the power supply around themanual switch SW, while the disc 36 is making one complete revolution.When the cam lobe 46 returns to the position shown in the drawing, thecam follower 76 is shifted radially outward, disconnecting the motorfrom the power supply by opening contacts 70 and 74, and simultaneouslyclosing contacts 72 and 74 to energize the solenoid 56 and raise thepositioning fork 48.

Thus, the manual operation of the slide-changing mechanism is such that,when the manual switch SW, is momentarily depressed, the solenoid 56 isdeenergized to pivot the positioning fork 48 out of engagement and themotor 38 is connected to the power supply independently of the manualswitch. The motor 38 then drives the slide-changing mechanism 10 to moveanother slide 12 into position. Thereafter, the motor 38 is disconnectedfrom the power supply and the solenoid 56 reconnected to pivot the fork48 into positioning engagement with the new slide 12.

It will be seen that the operation of the cam switch SW is exactly thesame regardless of the direction of rotation of the crank disc 36.Therefore, provision may be made for reversing the direction of movementof the slides 12 by simply reversing the polarity of the voltagesapplied to the motor 38. This may be accomplished simply by providing apolarity reversing switch (not shown) for the motor 38.

In accordance with the present invention, the slide-changing mechanism10 and the associated control components are combined in a novel mannerwith a timing control circuit, indicated generally at 77, for automaticcontrol of the dwell time between successive, automatically initiatedslide-changing cycles, and to permit selective and rapid adjustment ofthe length of the cycle within a relatively wide range, all withconventional, relatively inexpensive components that may be assembledand installed as a very compact and reliable control unit. The existingcontrol components previously described in connection with the manualoperation of the slide-changing mechanism are utilized in the automatictimer as well, the cam 46 and the cam-operated switch SW, serving toinitiate the timing cycle as the cam moves into the dwell position shownin the drawing, to reset the circuit during each automatic slidechangingoperation, and to initiate successive, timed dwell periods aftersuccessive slide changes.

To these ends, the automatic timer utilizes a silicon-controlledrectifier SCR which is essentially connected in parallel with the manualslide-change switch SW, and serves the same function during automaticoperation as does switch SW, during manual operation. Timed triggeringof the controlled rectifier SCR is provided by a unijunction transistor,relaxation oscillator circuit which includes a potentiometer foradjustment of the timed dwell period for the slide projector bycontrolling the rate of change of a timing capacitor C,. The controlledrectifier SCR and triggering relaxation oscillator circuit areinterconnected with the cam-operated switch SW so that the normalswitching operation of switch SW controls the power applied to thecontrolled rectifier SCR and oscillator to initiate timed dwell periodsand reset the timing control circuit. The automatic timer is selectivelyconnected to the slide-changing mechanism 10 by means of a manuallyoperable on-off switch SW lt will be evident that the timing cycle takesplace while the motor 38 and the crank disc 36 are at rest with the camlobe 46 in engagement with the follower 76, the belt 16 thus remainingstationary with a slide 12 in the projection position as shown in thedrawing. The controlled rectifier SCR is triggered, after the selecteddwell interval, by the relaxation oscillator, thereby creating aconducting path between the negative terminal 66 of the motor 38 and thenegative side 64 of the power supply, turning the motor on to begin theslidechanging cycle. The conducting path is through a diode D,, theanode-cathode circuit of the controlled rectifier SCR, the on-off switchSW, and the second and center contacts 72 and 74 of the cam switch SW,to the negative side 64 of the power supply.

When the motor 38 starts, the crank disc 36 turns the cam lobe 46 awayfrom the follower 76, and the center contact 74 of the cam switch SW isdisconnected from the second contact 72, thereby disconnecting thecathode of the controlled rectifier SCR, as well as the solenoid 56,from the negative side 64 of the power supply. The cathode of thecontrolled rectifier and the common point 78 of the timing circuit arethen connected to the positive side 58 of the power supply through therelatively low electrical impedance of the coil of the solenoid 56. Therectifier SCR and the relaxation oscillator circuit are then effectivelyshort-circuited through the solenoid coil to commutate the rectifier SCRand discharge the components of the relaxation oscillator circuit to aninitial condition of substantially 0 volts. A capacitor C is providedacross the timing circuit to aid in maintaining a steady directcurrentvoltage during both the timing and slide-changing periods as well as tofilter out any remaining alternating current voltages in the powersupply.

The controlled rectifier SCR and oscillator remain connected across thesolenoid coil, without power, throughout the slide-changing cycle,thereby preventing the normal, free running operation of the relaxationoscillator circuit. It will be seen, therefore, that the incorporationof the existing switching components of the mechanism 10 in the timingcircuit of the present invention permits the use of a relatively simplecontrolled-switch circuit to provide for automatic operation of themechanism.

As the slide-changing cycle is completed, the cam lobe 46 on the crankdisc 36 moves the cam follower 76 back into the dwell position shown inthe drawing, and thus connects the solenoid 56 and timing circuit to thenegative side 64 of the power supply and, concurrently, disconnects themotor 38 from the power supply. The cam lobe 48 thus moves the camfollower 76 into position for projecting a slide 12.

Herein, the unijunction transistor relaxation oscillator circuit has aresistor R, connected between the positive side 58 of the power supplyand base-two of the unijunction transistor 0,. A second resistor R isconnected between base-one of the unij unction transistor and the commonpoint 78 for the timing circuit at the cathode of the controlledrectifier SCR. In the usual manner, the gate electrode of the controlledrectifier SCR is connected to the junction of base-one of theunijunction transistor 0, and to the resistor R One end of a linearpotentiometer R is connected to the positive side 58 of the power supplyand the center tap and the other end of the potentiometer are connectedthrough a resistor R, to the emitter of the unijunction transistor 0,.The timing capacitor C, is connected between the emitter of theunijunction transistor Q, and the common point 78.

The relaxation oscillator circuit functions in the usual manner in thatthe resistors R, and R provide a bias potential between .the base-oneand the base-two of the unijunction transistor 0,. Potentiometer Rresistor R, and capacitor C, form a charging circuit which provides asubstantially linear voltage buildup across capacitor C, until thevoltage between the emitter and the base-one of the unijunctiontransistor Q reaches the threshold point determined by the intrinsicstandoff ratio and the bias voltage for the unijunction transistor. Theunijunction transistor then fires, developing a voltage pulse acrossresistor R which triggers the controlled rectifier SCR to start themotor 38.

To vary the length of the timed interval, the length of time requiredfor the voltage buildup is varied by adjusting the potentiometer R andthereby changing the rate of charging of the timing capacitor C,. Byreducing the charging rate, the interval is increased, and by increasingthe rate, the interval is reduced. With the illustrated embodiment, ithas been found that the viewing time for a slide can be variedconveniently between about 2 and 60 seconds.

A current-hold network is provided to prevent commutation, or turningoff, of the controlled rectifier SCR because of circuit interruptionsdue to the commutator of the motor. In the present instance, thisnetwork comprises a resistor R connected in series with a diode Dbetween the positive side 58 of the power supply and the anode of thecontrolled rectifier SCR. Thus, when the controlled rectifier fires, asmall holding current is provided through resistor R and diode D tomaintain the controlled rectifier in an ON condition regardless of theintermittent currents through the motor 38.

it should be noted that, even though the on-ofi switch SW; for thetiming circuit has been closed to initiate timed automatic operation,the manual switch SW remains efiective to initiate a slide change duringthe timing of a selected interval by the timing circuit. Upon manualclosure of switch SW,, the motor 38 is immediately energized to startits slide-changing cycle, terminating the partially timed interval andpreparing the circuit for timing of the next interval when a new slideis in the projection position.

Typical component values for the timing circuit and associated elementsare:

Power supply 24 volts DC Motor 24 volts DC motor for developing a camand disc angular velocity of approximately 30 r.p.m. Solenoid 24 volt DCsolenoid Q. ZN4871 SCR 2N506O D, lN400l D, lN400l C 50 mierot'arads 50volts C, 50 microl'arads 50 volts R. 560 ohms R 27 ohms R, l megohmlinear potentiometer R, 33 K R, L000 ohms From the foregoing, it will beseen that the present invention combines a slide-changing mechanism ofthe general type disclosed in the aforesaid patent with a timing circuitof conventional form, and interrelates the elements of the mechanism andof the circuit to produce a novel automatic time for a slide projector,the timer being simple, compact and inexpensive in construction andhighly effective and reliable in operation. While a particular preferredembodiment of the timing circuit has been described and illustrated foruse with a particular motor-driven slide-changing mechanism 10, it willbe appreciated that many variations and modifications are within thespirit and scope of the invention.

I claim as my invention:

1. In a slide-changing mechanism for use in a photographic slideprojector having an electric motor for driving the slidechangingmechanism, the motor being activated by a direct current power supply tochange a slide and deactivated when a slide is being shown, and havingan electric solenoid-operated positioning fork with the solenoid beingenergized to hold a slide in position when the motor is deactivated andthe solenoid deenergized to release the slide when the motor isactivated to change a slide, a timer circuit for said mechanismcomprising:

a single-pole, double-throw switch having center, first and secondcontacts, said center contact being electrically connected to saidsecond contact when the motor is deactivated and connected to said firstcontact after the motor is activated, said center contact beingconnected to the negative side of the power supply;

a silicon-controlled rectifier having an anode, a cathode and a gateelectrode, said cathode being connected to said second contact of saidswitch and one side of the solenoid coil, said anode being connected tosaid first contact of said switch and to the negative terminal of themotor, and the other side of the solenoid coil and the positive terminalof the motor being connected to the positive side of the power supply;and a triggering circuit for said rectifier including a unijunctiotransistor having first and second bases and an emitter, a biasingcircuit having a first resistor connected between the positive side ofthe power supply and said second base of said unijunction transistor anda second resistance connected between said first base of saidunijunction transistor and the negative side of the power supply, saidgate electrode of said rectifier being connected to said first base ofsaid unijunction transistor, and a biasing circuit having third andfourth resistors connected in series between the positive side of thepower supply and said emitter of said unijunction transistor and acapacitor connected between said emitter and the negative side of saidpower supply.

2. A timer circuit as defined in claim 1 including a holding circuitincluding a fifth resistor and a first diode connected in series betweenthe positive side of the power supply and said anode of said rectifierfor supplying a holding current through said rectifier after it istriggered.

3. A timer circuit as defined in claim 1 including a second diodeconnected between said first contact of said single-pole double-throwswitch and said anode of said rectifier.

4. A timer circuit as defined in claim 3 including a holding circuithaving a fifth resistor and a first diode connected in series betweenthe positive side of the power supply and said anode of said rectifierto provide a holding current through said rectifier after it istriggered.

5. A timer circuit as defined in claim 1 wherein said third resistor insaid timing circuit is variable.

6. In a slide-changing mechanism for use in a photographic slideprojector having an electric motor for driving the mechanism, the motorbeing activated by a direct current power supply to change a slide anddeactivated while a slide is being shown, a timer circuit comprising:

controlled switch means for activating the motor a predetermined timeperiod after the motor is deactivated, said controlled switch meanshaving a controlled rectifier connected between the power supply and themotor and triggering means connected to said rectifier for triggeringsaid rectifier after said predetermined time period, said triggeringmeans being connected to the power supply said reset switch meansdisconnecting said rectifier from the power supply and the motor afterthe motor is activated, said reset switch means thereafter substantiallyshort-circuiting the anode to cathode circuit of said rectifier anddisconnecting the triggering means from the power supply andsubstantially short-circuiting the power input terminals of saidtriggering means to commutate said rectifier and reset said triggeringmeans.

7. In a slide-changing mechanism for use in a photographic slideprojector having an electric motor for driving the mechanism, the motorbeing activated by a direct current power supply to change a slide anddeactivated while a slide is being shown, a timer circuit comprising:

controlled switch means for activating the motor a predetermined timeperiod after the motor is deactivated, said controlled switch meanshaving a controlled rectifier connected between the power supply and themotor and triggering means connected to said rectifier for triggeringsaid rectifier after said predetermined time period, said triggeringmeans being connected to the power supply through a pair of power inputterminals, said triggering means having a unijunction transistorconnected in a relaxation oscillator circuit, said oscillator circuithaving a biasing network and a charging network connected to the powersupply through a pair of power input terminals; and

reset switch means connected to said controlled switch means forresetting said controlled switch means after the motor is activated andmaintaining said controlled switch means in a reset condition until themotor is deactivated, said reset switch means disconnecting saidrectifier from between the power supply and the motor after the motor isactivated, said switch means thereafter substantially short-circuitingthe anode to cathode circuit of said rectifier and disconnecting therelaxation oscillator from the power supply and substantiallyshort-circuiting said power input terminals thereof to commutate saidrectifier and reset said relaxation oscillator circuit to predeterminedinitial conditions.

8. A timer circuit as defined in claim 7 including:

an electrical impedance; and

wherein said reset switch means includes a single-pole double-throwswitch having first and second outer contacts and a center contact beingconnected to said second contact when the motor is deactivated andconnected to said first contact after the motor is activated, the centercontact of said single-pole, double-throw switch being connected to thenegative side of the power supply;

wherein the positive side of the power supply is commonly connected toone side of said electrical impedance, one of said power terminals ofsaid relaxation oscillator circuit and the positive input terminal ofthe motor;

wherein said second contact of said single-pole, doublethrow switch iscommonly connected to the cathode of said controlled rectifier, theother power input terminal of relaxation oscillator circuit and theother side of said electrical impedance; and

wherein said first contact of said single-pole, double-throw switch iscommonly connected to the negative terminal of the motor and the anodeof said rectifier.

9. A timer circuit as defined in claim 8 wherein said electricalimpedance is a solenoid coil.

1. In a slide-changing mechanism for use in a photographic slideprojector having an electric motor for driving the slide-changingmechanism, the motor being activated by a direct current power supply tochange a slide and deactivated when a slide is being shown, and havingan electric solenoid-operated positioning fork with the solenoid beingenergized to hold a slide in position when the motor is deactivated andthe solenoid deenergized to release the slide when the motor isactivated to change a slide, a timer circuit for said mechanismcomprising: a single-pole, double-throw switch having center, first andsecond contacts, said center contact being electrically connected tosaid second contact when the motor is deactivated and connected to saidfirst contact after the motor is activated, said center contact beingconnected to the negative side of the power supply; a silicon-controlledrectifier having an anode, a cathode and a gate electrode, said cathodebeing connected to said second contact of said switch and one side ofthe solenoid coil, said anode being connected to said first contact ofsaid switch and to the negative terminal of the motor, and the otherside of the solenoid coil and the positive terminal of the motor beingconnected to the positive side of the power supply; and a triggeringcircuit for said rectifier including a unijunction transistor havingfirst and second bases and an emitter, a biasing circuit having a firstresistor connected between the positive side of the power supply andsaid second base of said unijunction transistor and a second resistanceconnected between said first base of said unijunction transistor and thenegative side of the power supply, said gate electrode of said rectifierbeing connected to said first base of said unijunction transistor, and abiasing circuit having third and fourth resistors connected in seriesbetween the positive side of the power supply and said emitter of saidunijunction transistor and a capacitor connected between said emitterand the negative side of said power supply.
 2. A timer circuit asdefined in claim 1 including a holding circuit including a fifthresistor and a first diode connected in series between the positive sideof the power supply and said anode of said rectifier for supplying aholding current through said rectifier afTer it is triggered.
 3. A timercircuit as defined in claim 1 including a second diode connected betweensaid first contact of said single-pole double-throw switch and saidanode of said rectifier.
 4. A timer circuit as defined in claim 3including a holding circuit having a fifth resistor and a first diodeconnected in series between the positive side of the power supply andsaid anode of said rectifier to provide a holding current through saidrectifier after it is triggered.
 5. A timer circuit as defined in claim1 wherein said third resistor in said timing circuit is variable.
 6. Ina slide-changing mechanism for use in a photographic slide projectorhaving an electric motor for driving the mechanism, the motor beingactivated by a direct current power supply to change a slide anddeactivated while a slide is being shown, a timer circuit comprising:controlled switch means for activating the motor a predetermined timeperiod after the motor is deactivated, said controlled switch meanshaving a controlled rectifier connected between the power supply and themotor and triggering means connected to said rectifier for triggeringsaid rectifier after said predetermined time period, said triggeringmeans being connected to the power supply through a pair of power inputterminals; and reset switch means connected to said controlled switchmeans for resetting said controlled switch means after the motor isactivated and maintaining said controlled switch means in a resetcondition until the motor is deactivated, said reset switch meansdisconnecting said rectifier from the power supply and the motor afterthe motor is activated, said reset switch means thereafter substantiallyshort-circuiting the anode to cathode circuit of said rectifier anddisconnecting the triggering means from the power supply andsubstantially short-circuiting the power input terminals of saidtriggering means to commutate said rectifier and reset said triggeringmeans.
 7. In a slide-changing mechanism for use in a photographic slideprojector having an electric motor for driving the mechanism, the motorbeing activated by a direct current power supply to change a slide anddeactivated while a slide is being shown, a timer circuit comprising:controlled switch means for activating the motor a predetermined timeperiod after the motor is deactivated, said controlled switch meanshaving a controlled rectifier connected between the power supply and themotor and triggering means connected to said rectifier for triggeringsaid rectifier after said predetermined time period, said triggeringmeans being connected to the power supply through a pair of power inputterminals, said triggering means having a unijunction transistorconnected in a relaxation oscillator circuit, said oscillator circuithaving a biasing network and a charging network connected to the powersupply through a pair of power input terminals; and reset switch meansconnected to said controlled switch means for resetting said controlledswitch means after the motor is activated and maintaining saidcontrolled switch means in a reset condition until the motor isdeactivated, said reset switch means disconnecting said rectifier frombetween the power supply and the motor after the motor is activated,said switch means thereafter substantially short-circuiting the anode tocathode circuit of said rectifier and disconnecting the relaxationoscillator from the power supply and substantially short-circuiting saidpower input terminals thereof to commutate said rectifier and reset saidrelaxation oscillator circuit to predetermined initial conditions.
 8. Atimer circuit as defined in claim 7 including: an electrical impedance;and wherein said reset switch means includes a single-pole double-throwswitch having first and second outer contacts and a center contact beingconnected to said second contact when the motor is deactivated andconnected to said first contact after the motor is actiVated, the centercontact of said single-pole, double-throw switch being connected to thenegative side of the power supply; wherein the positive side of thepower supply is commonly connected to one side of said electricalimpedance, one of said power terminals of said relaxation oscillatorcircuit and the positive input terminal of the motor; wherein saidsecond contact of said single-pole, double-throw switch is commonlyconnected to the cathode of said controlled rectifier, the other powerinput terminal of relaxation oscillator circuit and the other side ofsaid electrical impedance; and wherein said first contact of saidsingle-pole, double-throw switch is commonly connected to the negativeterminal of the motor and the anode of said rectifier.
 9. A timercircuit as defined in claim 8 wherein said electrical impedance is asolenoid coil.